1. Field of the Invention
The present invention relates to a travelling vehicle, and more particularly relates to a travelling vehicle in which a plurality of servomotors are used as travel motors.
2. Description of the Related Art
An example of a known conveyance vehicle that performs automatic travel is a stacker crane used in an automated warehouse.
For example, a conventional automated warehouse has a pair of racks provided a specific distance apart in a front and back direction, a stacker crane provided movably in the left and right direction between the racks, an unloading station located to the side of one of the racks, and a loading station located to the side of the other rack. The racks have numerous article holding shelves extending in the vertical and horizontal directions. The stacker crane has a travelling vehicle, a hoist that can move up and down a mast provided to this vehicle, and a article movement apparatus provided thereto (such as a slide fork provided so as to slide back and forth) (see, for example, Japanese Laid-Open Patent Application 2008-100773).
With the stacker crane described in Japanese Laid-Open Patent Application 2008-100773, a total of four servomotors are used as travel motors.
In the past, a simple power supply cut-off was performed to actuate the mechanical brakes of the servomotors and thereby stop the stacker crane. However, in multi-axis travel using a plurality of travel-use servomotors, it was difficult to balance the torque of the mechanical brakes of the servomotors, and when the brakes were actuated on all axes at the same time, the mechanical structure was sometimes subjected to an excessive load. On the other hand, a stacker crane moves at high speed and high acceleration, and therefore the control deceleration torque produced by the servo drivers is higher than the deceleration torque obtained with the mechanical brakes of the servomotors.
One possible method for dealing with this problem is to shut off the power after stopping under deceleration control by the servo drivers (stopping category 1). However, if a malfunction occurs in a servo driver, it may not be possible to stop under deceleration control.